1. Field of the Invention
The present invention relates to a polyhydroxyalkanoate synthesizing microorganism and a process of producing polyhydroxyalkanoate using the same.
2. Related Background Art
It has heretofore been reported that many microorganisms produce poly-3-hydroxy-n-butyric acid (hereinafter, sometimes referred to as “PHB” for short) and accumulate it in the bacterial cells (cf., Biodegradable Plastics Research Group, ed, “Biodegradable Plastics Handbook”, NTS INC., p 178-197 (1995)). The polymers, like conventional plastics, can be used for the production of various products by melt processing or the like. Further, because of their biodegradability, they are advantageous in that they can be completely degraded by microorganisms in nature. Unlike most conventional synthetic high polymer compounds, few of them remain in the natural environment to cause environmental pollution.
PHB is a kind of “usual-PHA”, that is, PHA composed of a monomer unit having an alkyl group in the side chain. On the other hand, taking into consideration wide application of PHA, for example, application as a functional polymer, “unusual-PHA”, which is PHA that has introduced in the side chain a substituent other than alkyl groups is expected to be very useful. Examples of the substituent include those substituents that contain an aromatic ring and unsaturated hydrocarbons. PHAs can be produced by culturing microorganisms while feeding as substrates them with substituted alkanoates or substituted alkanes having substituents to be introduced.
For example, Japanese Patent Application Laid-Open No. 2002-80571 discloses the production of unusual-PHAs by various microorganisms including Pseudomonas cichorii YN2 strain, Pseudomonas cichorii H45 strain, Pseudomonas jessenii P161 strain, Pseudomonas putida P91-strain using as the substrates substituted alkanoates having a phenyl group, a phenoxy group or a cyclohexyl group in the side chain. Further, Japanese Patent Application Laid-Open No. 2003-310292 discloses the production of unusual-PHAs by Pseudomonas cichorii YN2 strain using as substrates various substituted alkanes having an aromatic ring in the side chain.
While as stated above, functionally useful unusual-PHAs are produced by some microorganisms, it is essential to decrease production cost of PHAs than ever in order to apply the unusual-PHAs to a wide variety of products as high polymer materials. For this purpose, one issue is to increase production efficiency of unusual-PHAs by microorganisms and another is to use less expensive substrates than ever in the production of unusual-PHAs. In particular, the substrates necessary for the production of PHAs in many cases are chemically synthesized and their production cost of PHAs largely depends on the price of the substrates. The substituted alkanoates, which are generally used as substrates, contain a carboxylic group having activity, so the production of PHAs with substituted alkanoates in most cases requires cumbersome operations such as protection and deprotection of carboxylic groups. This means an increase in the number of process steps and hence a higher price. On the other hand, when substituted alkanes are used as substrates, the number of process steps is smaller than that in the case of using the substituted alkanoates as substrates, so the price is relatively low. However, in most cases, the productivity of PHAs by the microorganisms is lower and the culturing method is more complicated than the case in which the substituted alkanoates are used as substrates. From the foregoing, it is desired that higher-productivity than ever be obtained by using substituted alkanes as substrates to produce PHAs at low cost.
Further, there has been a problem in that in the case of microorganisms with a high unusual-PHA productivity, for example, YN2 strain, the PHA productivity depends largely on the pH of the medium, that is, the PHA productivity decreases largely at pH values other than the optimum pH around pH7. In particular when the concentration of the substrate or medium is increased with a view to increase the productivity, pH tends to fluctuate beyond the buffering capability of the medium as culture proceeds, so the PHA productivity is unstable. Accordingly, it has been desired to produce stably PHAs against pH fluctuation.